This invention relates to gas sensors, and more particularly but not exclusively, to electrochemical gas sensors.
Electrochemical gas sensors are in common use to warn of danger from toxic gases and may be used also to warn of fire outbreak. Existing electrochemical gas sensors have not always failed safe; that is they have not indicated that they have failed, or given a signal equivalent to that from a dangerous gas concentration, when they are no longer operational.
Reliability of existing gas sensors has generally therefore only been ascertained by regular tests involving exposure to a calibration or test gas. The concentration of the test gas in the vicinity of the sensor must be known accurately and this often involves use of a considerable quantity of gas and of special methods to ensure the concentration is constant and reproducible. Particularly in fixed installations, such testing can be awkward and expensive and can necessitate temporary disabling of the system, which itself is undesirable because there is a risk that the system may not be switched on again. An electrochemical gas sensor whose function can be checked while responding to a signal gas, or with a remotely-controllable self-test capability, is therefore highly desirable.
Gas sensors with self-test by generation of a gas have been described in UK Patent GB 1,552,535 (Bayer Ltd.). The self-test sensor arrangement described comprises two elements: a sensor and a gas generation means, for example an electrolysis cell, joined to the sensor by a delivery channel. Test gas, generated by the cell, is delivered to the input of a diffusion limited gas sensor, with a membrane between the point of gas delivery and the outside world. Delivery is by a piston, a pressure difference resulting from the generation of gas itself or other means. Signal gas approaches the gas sensor from the outside world via the membrane. It is not apparent whether the membrane is intended to be a diffusion barrier, whose permeability controls the response of the sensor, or whether the membrane has a high permeability and acts mainly to prevent contamination of the apparatus by, for example, airborne dust.
In the aforementioned UK Patent, the concentration of test gas detected by the sensing electrode depends on the balance of rate of generation of the gas and the rate of loss of gas through the membrane. The latter depends on a number of conditions (including: air flow, humidity and temperature) outside the membrane. Unless a large amount of test gas is generated, or the membrane has very low permeability, the response from the sensing electrode under test depends on air flow in an undesirable way. However, a very low permeability of the diffusion barrier causes the sensor to have low sensitivity to the signal gas and so might itself be undesirable. The arrangement described in UK Patent GB 1,552,538 therefore needs a comparatively large amount of gas to give reliable operation; in turn requiring significant energy, which limits its usefulness, in particular in situations where only low power is available.
A possible failure mode of fixed installation gas sensors, incorporating the aforementioned arrangement, is that gas access from the atmosphere might become partially or completely blocked. Delivery of test gas inside the cell might result in a rise in response above that expected, as the rate of escape is reduced. However, this only becomes apparent if the sensor response is dependent entirely on the concentration of test gas inside the sensor. As the test gas is delivered directly to the sensing electrode in the aforementioned arrangement, the electrode response to the test gas is likely to be limited partially by the activity of the electrode itself, and is therefore unlikely to give a reliable indication of a blocked outer barrier.
An aim of the invention is to provide an improved gas sensor which allows reliable checks on its performance to be made and overcomes at least some of the aforementioned problems.
According to the present invention there is provided a gas sensor comprising: a sensing cell and a test cell, the test cell being arranged, so that in use, a test gas is generated on demand and a pathway is provided for directing said test gas from the test cell to the sensing cell, there being at least first and second sensing electrodes in the sensing cell, characterised in that the test gas is directed so that at least two electric currents are generated and means is provided for comparing the currents in order to provide a value indicative of the status of at least one electrode.
The life of the test gas generator depends on the amount of test gas needed in each test; this could be prohibitive in a sensor intended for long life or for frequent test if the desired amount of test gas is large. The present invention reduces considerably the amount of test gas required and makes feasible the use of small test gas generators which store small amounts of consumable substance.
Advantageously electrochemical gas generation of, and subsequent detection of, a test gas is achieved in a manner which uses less energy than previous devices.
Inclusion of a test gas generator cell and sensing cell in the same housing reduces the volume of test gas needed and hence the energy needed to power the test gas generator. Furthermore delivery of test gas between two diffusion barriers allows an increase in sensitivity above a certain threshold. This enables a warning of a blocked gas access from outside the cell to be provided.
Substantially planar cell design allows sensing and generator electrodes to be fabricated on the same substrate at a single stage. This reduces cost and allows closer spacing of components. In practice the volume of test gas required is reduced and this provides a more responsive sensor.
According to a second aspect of the present invention there is provided a sensing cell with two sensing electrodes, with gas access to the second cell via the first cell, there being a known diffusion impedance between the said cells, each cell, in use, generates an electric current, the ratio of which currents, in response to a signal gas from the atmosphere, is used to provide an indication of activity of the electrodes.
The aforementioned arrangement may be used to derive a calibration coefficient for the cell, hence maintaining its calibration beyond the point at which a conventional cell would have failed. Most importantly if the ratio reaches a predetermined value a warning of impending failure of the cell is provided. This may be activated automatically by suitable referencing means such as an integrated circuit.
Combination of a test gas generator cell and a sensor cell allows decay of the sensing electrodes to be detected using test gas when no signal gas is present. This can be used to derive a calibration coefficient. Such an embodiment is less sensitive than a conventional arrangement, or of cells with sensor and generator in the same housing, but only one sensing electrode is required. The electrode senses variations in concentration of test gas resulting, for example, from variation in performance of the test gas generator, or from variation in external air velocity, thereby allowing smaller volumes of test gas to be used, and therefore requires less energy to generate the gas.
If a second gas generator is used, which delivers gas to the second sensing electrode, together with a first gas generator which delivers gas to the first sensing electrode, each gas subsequently being able to diffuse to the other sensing electrode, then comparison of the electrode response to gas from two gas generators allows measurement of the decay of each of the two electrodes to be made. This enables an even more accurate estimate of the calibration coefficient to be made than if one generator is used alone.
Preferably sensing and generator electrodes are in the same housing and arranged so that gas is delivered from the generator electrode into a space between two diffusion barriers, the first diffusion barrier leads gas from the outside atmosphere into the space, the second barrier leads gas from this space to the sensing electrode.
According to a third aspect of the present invention there is provided a sensing cell with two sensing electrodes, with gas access to the second cell via the first cell, there being a known diffusion impedance between the said cells, each cell, in use, generating an electric current, the ratio of which currents, in response to a signal gas from the atmosphere, is used to provide an indication of the status of the cell.
In a preferred embodiment electric current is generated by a sensing electrode in response to the test gas being limited by diffusion through the second barrier. This in turn ensures that the electric current is determined by the concentration of gas in the intermediate space, and hence on the rate of diffusion of test gas from the space through the first barrier to outside the cell. If the first barrier becomes blocked by contaminants, the electric current generated in response to test gas rises. This can be used as a warning that blockage has occurred. If the second barrier were not present then electric current would depend partially on the electrode parameters and would not necessarily rise above its usual value if a blockage occurred. This preferred feature of the invention is considered to be of particular importance given the fact that gas sensors may be used in dusty environments.
The sensing cell preferably has two sensing electrodes with a connecting gas pathway therebetween. Signal gas from the outside world (atmosphere) arrives first in the vicinity of the first electrode and then (if not reacted at the first electrode) passes to the second electrode. The sensing cell contains electrolyte, a counter electrode and optionally a reference electrode. The two sensing electrodes are connected to separate current measuring devices and may be biased at either the same or different potentials, with respect to the electrolyte. Signal gas entry from the outside world to the connecting passage, is via a first diffusion barrier which controls the response of the sensing cell. Optionally a second diffusion barrier might be placed in the passage between the first and second sensing electrodes.
In normal operation, the areas and activities of the sensing electrodes and the diffusional impedance between the first and second sensing electrodes determines a proportion of the signal gas which reacts at each electrode, and hence a ratio of the currents from each. If the area of the first electrode is large and the diffusional impedance between first and second electrodes is large, most of the current will be generated at the first electrode. The sum of the currents is determined by the factors above and also by the diffusional impedance of the first (external) diffusion barrier. The current is also proportional to the external signal gas concentration. If the activity of the sensing electrodes decays, the proportion of the gas which reacts at each electrode will changexe2x80x94in general, the proportion at the second electrode will increasexe2x80x94and this will be shown by a change in the ratio of the currents.
As the electrodes decay and the current ratio changes the summed current tends to fall because gas reacting at the second electrode has diffused through a greater impedance to reach this electrode than will the gas which reacts at the first electrode. This additional diffusional impedance acts in the same way as an increase in the impedance of the external barrier in a conventional cell and the cell calibration coefficient changes. The cell therefore becomes less sensitive. However, in principle for any ratio of currents the effect of the additional impedance is known and so the new correct calibration coefficient can be found. Therefore once the cell has been calibrated initially, it can be recalibrated through the process of measurement of signal gas, even though the concentration of the signal gas is not known. Only the ratio of electrode currents is needed.
If signal gas were regularly present and there were no safety implication, then observation of the current ratio indicates correct operation of the sensor. In applications such as fixed site safety or fire monitoring, signal gas is not present except during an alarm or test procedure. Such installations are tested regularly by applying a known concentration of test gas to the sensor manually. In a particularly advantageous embodiment, the test depends only on the ratio of responses from two sensing electrodes, rather than the absolute response, so the concentration of signal gas used in the test need not be known. Therefore measures to ensure a known concentration in the vicinity of the sensor are not needed. This is advantageous for example if the detector location makes access difficult.
The sensing cell may be combined with a test gas generator in the same housing. This combination gives a self-test sensor which requires lower power and is more accurate than currently available devices.
A test gas generator cell is preferably included in the sensor housing so that a test gas generator outlet communicates with the gas space in a region adjacent the first sensing electrode in such a way that the test gas first reaches the first sensing electrode and then the second electrode. The gas generator cell may include means which liberates a controlled quantity of test gas in response to an external signal, usually electrical, e.g. an electrolysis cell which generates hydrogen from a moist electrolyte. The ratio of currents from the electrodes in response to an amount of test gas can then be used to calibrate the cell in the same way as above. The need for a known concentration of test gas in this space, mentioned above for present self-test sensors, is now relaxed, making calibration of the sensor more immune to external effects (e.g. drafts and moving air). Much smaller concentrations of test gas are required, which further reduces the power consumption.
Other preferred features of the invention are expressed in the dependent claims.